This invention relates to a large vacuum chamber system, a new method for creating a vacuum in a large volume, systems utilizing the chamber and method for vacuum food processing and pyrolysis of hydrocarbon containing material.
Many processes and procedures need a vacuum to produce desired results. Other processes would benefit if carried out in vacuum. In freeze-drying of food, for example, a vacuum is used to lower the boiling point of the water in the food so that it can be removed from the food at low temperature. The freeze-drying of most foods, such as coffee, can be accomplished in a relatively short time so that a relatively small chamber may be sufficient for vacuum processing of all of a company's production. Where the vacuum processing takes longer, the total volume for vacuum processing must be increased to accommodate the extra material held in the processing system.
Other processes involving gas flows are pressure dependent. For example, the pyrolysis of hydrocarbon containing material such as oil shale or tar sands can be enhanced by reduced pressure. In the standard above-ground retort shale or tar sand is heated to a pyrolysis temperature thermally cracking the hydrocarbons. Some fractions such as methane are released as uncondensible vapor, and those that form condensates ultimately end up as petroleum products.
The vapor that is created must be removed rapidly. Because of the surrounding cloud of hydrocarbon vapors, each having its own partial pressure, it is more difficult for molecules to escape from the rock or other material being pyrolyzed because the molecules that are attempting to escape collide with reverse flux molecules in the surrounding cloud, and at ambient pressure, most molecules reflect back toward the source material. This slows the process rate so that for a given retort having a certain capital cost, the amount of material that can be processed in a given time decreases. If the hydrocarbon molecules could be removed from the material at molecular speeds up to thousands of feet per second rather than the mass transfer speeds of inches or a few feet per second, the process may be dramatically speeded up. In a vacuum, molecules travel at their molecular speeds because they seldom collide with neighboring molecules. This speed is a function of the system temperature and in an inverse relationship to the pressure.
For economical pyrolysis, large scale systems are believed necessary. If the amount of material processed by a system increases without a corresponding increase in capital costs, the material produced by the latter system will cost less because it has a smaller capital cost factor. Vacuum systems, however, have inherently been limited in size for reasons set forth more fully below. If a large vacuum retort can be built, pyrolysis costs could be reduced depending on the process's sensitivity to pressure and the capital costs associated with implementing the vacuum.
It will be recognized that large vacuum chambers have other uses. Some of the largest vacuum chambers in existence today were developed in conjunction with the aerospace program so that space vehicles could be tested to simulate the vacuum that exists above the atmosphere. For example, the early manned space vehicles were tested in the large vacuum chambers to determine if they leak and to ascertain heat transfer at low pressure. Manned space vehicles such as the space shuttle are orders of magnitude bigger than the earlier space probes such as Gemini and Apollo, but there have been no large vacuum chambers built to accomodate the current larger vehicles. One of the largest known vacuum tanks in the United States is 40 feet (12.1 m) in diameter built at very great cost by NASA.
As size increases, the total force acting on the walls of the chamber increases so the wall thickness must increase disproportionately with volume. Therefore, the main limitation on the size of vacuum chambers is the wall thickness necessary to sustain the collapsing forces.
The pressure acting on the outside wall of the chamber is a function of the following equation: ##EQU1## where p=pressure differential between the inside and the outside of the chamber wall (psi);
E=modulus of elasticity=3.0.times.10.sup.7 psi (stainless steel); PA1 t=wall thickness (in); PA1 l=length of unsupported cylinder (in); and PA1 r=tank radius (in). PA1 p=outside ambient pressure (psi); PA1 G=number of rotational gravities over one; and PA1 W=weight per unit area of wall (psi). EQU W=wt (3) PA1 w=density of the rotating material (lbs/in.sup.3); and PA1 t=wall thickness (in). PA1 K=a constant of proper units; PA1 D=tank diameter (in), and PA1 N=tank rotational speeds (rpm). PA1 D.sub.R =rotational drag; PA1 S=shell area=DL (ft.sup.2). PA1 .rho.=density of air=0.002378 slugs/ft.sup.3. PA1 D.sub.R =1509.12 lb.
With Equation (1), the pressure differential that a chamber can withstand may be calculated based on varying chamber radius and wall thickness. Although the theoretical pressure differential between the inside of the chamber at full vacuum and the outside will be 14.7 psi (7.1 g/sq. cm), a safety factor for the metal chamber wall must be considered. Assume that a tank is 15 ft (4.6 m) in diameter and has a 8 ft (2.4 m) length. If the wall thickness (t) is 0.5 in (12.7 mm) the tank will withstand a pressure differential of approximately 53.5 psi. Based on the 14.7 psi pressure differential that will be encountered, this is a safety factor of approximately 3.6 which is considered sufficient.
If the diameter of the tank is very much greater, say 60 ft (18.3 m) with a length of approximately 40 ft (12.2 m), the conditions radically change. Under equation (1) the same tank thickness of 0.5 in can withstand a pressure differential of only 1.34 psi. To withstand a pressure differential of 14.7 psi, the wall thickness would have to be increased to between 1 and 1.5 in, but that would yield no safety factor. To yield an effective pressure differential with the same safety factor of 3.6, the wall thickness would have to be approximately 2.25 in (5.72 cm). Such a tank made of stainless steel would weigh more than 360 tons (327 metric tons) without any supporting structure.
The fabrication of such a structure would also be extremely costly and impractical. Because of the size and weight of the tank, it must be built on site, and the fabrication equipment would have to be used at the site. The forming or welding of more than 2 inch thick plates of stainless steel into a cylindrical perfectly sealed tank on site is a most formidable task. It would be advantageous, therefore, to be able to build a vacuum chamber with very much thinner walls. As a comparison, a 10 ft.times.10 ft.times.3 in plate of stainless steel weighs up to 6 tons (5450 kg), but a 1/8 in thick plate of the same area weighs only about 500 lb (227 kg). Even though the large tank would still be built on site, it is relatively easy with today's technology to form and weld 1/8 in plate. A 60 ft in diameter tank of 40 ft length having a wall thickness of 1/8 in (3.2 mm) would weigh approximately 20 tons (18 metric tons) versus the 360 tons (327 metric tons) of the same tank with 2.25 in (50.4 mm) thick walls. The raw materials alone for the tank having the thicker walls cost about 18 times more than the thinner wall version for the same volume, and the fabrication costs would be more than 18 times greater for existing standard designs.
One of the objects, therefore, of the present invention is to disclose and provide a novel vacuum chamber which can use substantially thinner walls than was heretofore thought possible. Such a tank will substantially decrease the cost of manufacturing large vacuum chambers and allow processes not previously undertaken to be made feasible.
Another problem in vacuum processing is that the vacuum pumps must do more than merely remove the initial air in the chamber. As the pressure decreases, the material within the chamber evaporates or sublimes yielding molecules that must be removed from the chamber. The lower the internal pressure of the chamber that is sought, the greater the amount of the evaporating volumetric contents that must be removed. If the process requires near vacuum in the range of less than 0.5 in (11 mm) Hg, large amounts of energy must be expended to maintain the vacuum, and the cost of such energy may make an otherwise beneficial process uneconomical.
Another significant problem is that it is not desirable to pass the evacuants that contain condensables through the vacuum pumps. Doing so tends to make the pumps very much more inefficient because of condensation within the pump. The condensate also tends to damage the pump. As a result, elaborate systems have been developed to condense the condensables between the pumps and the chamber. These systems are quite costly, and they necessitate having double condensing equipment. When one condenser is filled with frozen condensate, the duplicate system must be used while the first system is defrosted. The condenser must be warmed for defrosting, and additional energy is necessary to cool the condenser again from its elevated temperature. This is inefficient and wasteful of energy.
Another problem with large vacuum chambers is the necessarily large conduits necessary to connect the vacuum pumps with the chamber. They must be of wide diameter to minimize the pressure drop between the pump and the chamber. The condensation problem, the necessary large diameter tubing and the duplication required substantially increases the cost of large vacuum processing systems such that it may be uneconomical to use an otherwise beneficial vacuum processing system.
The present system overcomes those problems dramatically.
Although some of the aforesaid problems are alleviated in the higher temperature environment of pyrolysis, others are created. In pyrolysis of shale and tar sands, the source material is reduced to a relatively small particle size through crushing or some other mechanical method or by explosions. By decreasing the particle size, the surface area increases and the reaction generally speeds up. The particle size should be an optimum of processing size for process time versus crushing and processing costs. If the particle size is too small, however, powder-like fines are created that impede processing by tending to contaminate the liquid condensate and vapor making the process substantially more difficult and contaminating the end product.
In combustion processes the heat for pyrolysis is obtained through combustion of some of the shale itself. The products of combustion tend to contaminate the end products. Whether the pyrolysis takes place in large, above ground retorts or in situ, because the rock is not of uniform size or hydrocarbon content, there is channeling and charring, which causes incomplete combustion and leads to further contamination of the desired hydrocarbons. Also, incomplete pyrolysis wastes shale. Both from a cost standpoint and a conservation viewpoint, it is advantageous to obtain all desired hydrocarbons from all processed ore such that the spent shale is environmentally innocuous.
Another potential problem in pyrolysis is that if the vapors are exposed to the atmosphere, they can become explosive and toxic. This problem has been solved at high cost in a number of different ways, but if processing occurs in a vacuum, no oxygen will support an explosion. Such a system can be a closed cycle, and all subsystems can be fully controlled.
As will be set forth more fully hereinafter, the process described is a closed cycle batch process. As with most batch processes, starting and stopping the cycle creates inefficiencies. The present invention overcomes many of these problems by rather unique solutions, and it is an object to disclose and provide an invention that eliminates many of the problems associated with a batch process. For example, most batch systems are energy inefficient because heat of process is lost and is not readily transferable to the next batch cycle. The present invention overcomes many of these problems.
Another problem may result in the use of processing time to raise the ore to pyrolysis temperatures. One object of the present invention is to save the heat from a batch that has been processed to use it on forthcoming batches. Another object of the present invention is to disclose and provide a system for loading and unloading a retort as quickly as possible so that time between batches is minimized. Another object of the present invention is to disclose and provide a system that minimizes the need for external condensers. Normally, it is desirable to maintain the majority of the hydrocarbons in the gaseous or vapor state until they reach the condenser because liquid shale oil or petroleum from tar sands is very heavy, viscous oil that tends to clog retort structure and to slow processing. One of the objects, therefore, of the present invention is to eliminate this necessity of maintaining the vapors at a high temperature until they reach the condenser. As will be discussed hereinafter, one of the advantages of the present invention is that the liquid phase can be reached efficiently within the chamber to the advantage of the system. One of the other objects of the present invention is that some of the heat of condensation can be utilized in preheating incoming ore. One of the other objects of the present invention is to provide structure that eliminates or removes the particulate matter that would have contaminated the liquid phase without additional equipment.